Well, firstly if I'm not mistaken, the value given for killed Soviet POW's is used in combination with the alleged figure of 11 million total deaths from the "Death" camps that Germany operated. These figures are likely to be just as unsubstantiated as the mythical six million figure for Jews.

As for German POW's, the figure I believe was retained only during the course of the war. It would be exponentially higher if we were to include former German troops that died within the Siberian gulags after the war.